1. Field of the Invention
The present invention relates to a lead cutting apparatus for cutting leads of electric and electronic devices and more specifically to a lead cutter which cuts leads or pins of electric and electronic devices and forms an anticorrosive coat at the cut ends and also to an anticorrosive coat structure of leads.
2. Prior Art
Conventional electric and electronic devices used for, for example, communication equipment are manufactured by the process described below.
FIGS. 6(a) through 6(c) are schematic diagrams which illustrate a manufacturing process of a dual in-line module.
As shown in these Figure, a lead frame 107 is comprised of a pair of connecting strips 103 and sets of pectinated leads 105 projected from the connecting strips 103 at appropriate intervals. An IC device 109 is conveyed by a belt 108 and is placed between a set of leads 105, and internal circuits (not shown) of the IC device 109 are electrically connected to the 105 by wire bonding. The IC device 109 is then embedded together with the leads 105 in a synthetic resin such as an epoxy by molding. The leads 105 are made of an alloy, and the surface of the leads is plated with, for example, tin in order to prevent rusting.
A molded IC device 101 is mounted on a lead cutter (not shown), and the leads 105 are cut at a predetermined portion, for example, as shown by the dotted lines in FIG. 6(c), so that each lead has an appropriate length.
FIGS. 7(a) and 7(b) show a conventional lead cutter 201 used for cutting and shaping the leads 105 of a molded IC device 101.
The lead cutter 201 is comprised of an upper die 203, a lower die 205, and a cutting blade 207. The upper die 203 is moved up and down in a vertical line, and the cutting blade 207 is also moved vertically along the side surface of the upper die 203. The cutting blade 207 has a flat-front end 207a at right angles relative to the longitudinal direction of the cutting blade 207 and has a cutting edge formed at the inside edge of the front end 207a.
In this conventional lead cutter, after the molded IC device 101 is mounted on the lead cutter 201, the upper die 203 is moved downward so that the under surface of the upper die 203 comes into contact with the upper surface of the lead 105 as shown in FIG. 7(b). A gap between the upper and lower dies 203 and 205 is predetermined so as to be almost the same as the thickness of the lead 105. As a result, the leads 105 are pressed and folded into a shape of a crank. Then, the cutting blade 207 is moved in the direction indicated by an arrow A to shear the leads 105.
After the leads 105 are cut, the cutting blade 207 and the upper die 203 are moved back to their initial positions, respectively, and the IC module 101 with the leads crank-shaped is dismounted from the cutter 201.
The lead cutter 209 shown in FIG. 8 has the same construction as the cutter 201 illustrated in FIGS. 7(a) and 7(b) except that the cutting blade 211 is moved upward in contact with the lower die 205 when the leads 105 are cut.
The IC modules 101 manufactured as described above is mounted on a printed circuit board (not shown), then the leads 105 are soldered to conductors on the printed circuit board.
However, according to the conventional lead cutters, the metallic material inside each lead 15 is exposed, as shown in FIGS. 9 and 10, at the cut ends 113 without the anticorrosive coat 111. Thus, the exposed metallic material may be rusted by the time the IC module 101 is mounted on a printed circuit board, and the rust formed on the cut ends causes the solderability of the lead to deteriorate and the reliability of soldering to decrease.
If the conventional lead cutter shown in FIG. 8 is used for cutting the leads of the molded IC chip 101, a little anticorrosive coat 111a may remain at the lower portion of the cut end 113 as shown in FIG. 10. The anticorrosive coat 111a, which contacts the conductor on the printed circuit board when the IC module is mounted on the printed circuit board, promotes a solderability of the leads 105. On the other hand, if the lead is cut by the conventional lead cutter shown in FIG. 7, a little anticorrosive coat 111a remains at the upper portion of the cut end 113 as shown in FIG. 9, and the anticorrosive coat 111a that exists at the upper portion of the lead does not contribute to soldering.
In view of the above, an anticorrosive treatment at the cut ends is required after the shaping and cutting of the leads are finished in order to prevent the cut ends of the leads from rusting. However, this anticorrosive treatment increases the number of the steps of the manufacturing process and causes the manufacturing cost to be higher.